There is an ongoing effort to develop systems that are more energy-efficient. A large proportion (some estimates are as high as twenty-five percent) of the electricity generated in the United States each year goes to lighting, a large portion of which is general illumination (e.g., downlights, flood lights, spotlights and other general residential or commercial illumination products). Accordingly, there is an ongoing need to provide lighting which is more energy-efficient.
Solid state light emitters (e.g., light emitting diodes) are receiving much attention due to their energy efficiency. It is well known that incandescent light bulbs are very energy-inefficient light sources—about ninety percent of the electricity they consume is released as heat rather than light. Fluorescent light bulbs are more efficient than incandescent light bulbs (by a factor of about 10) but are still less efficient than solid state light emitters, such as light emitting diodes.
In addition, as compared to the normal lifetimes of solid state light emitters, e.g., light emitting diodes, incandescent light bulbs have relatively short lifetimes, i.e., typically about 750-1000 hours. In comparison, light emitting diodes, for example, have typical lifetimes between 50,000 and 70,000 hours. Fluorescent bulbs have longer lifetimes (e.g., 10,000-20,000 hours) than incandescent lights, but provide less favorable color reproduction.
Another issue faced by conventional light fixtures is the need to periodically replace the lighting devices (e.g., light bulbs, etc.). Such issues are particularly pronounced where access is difficult (e.g., vaulted ceilings, bridges, high buildings, traffic tunnels) and/or where change-out costs are extremely high. The typical lifetime of conventional fixtures is about 20 years, corresponding to a light-producing device usage of at least about 44,000 hours (based on usage of 6 hours per day for 20 years). Light-producing device lifetime is typically much shorter, thus creating the need for periodic change-outs.
General illumination devices are typically rated in terms of their color reproduction, i.e., the extent to which objects illuminated by the illumination devices are perceived to be the color that they actually are. Color reproduction is typically measured using the Color Rendering Index (CRI Ra). CRI Ra is a modified average of the relative measurements of how the color rendition of an illumination system compares to that of a reference radiator when illuminating eight reference colors, i.e., it is a relative measure of the shift in surface color of an object when lit by a particular lamp. The CRI Ra equals 100 if the color coordinates of a set of test colors being illuminated by the illumination system are the same as the coordinates of the same test colors being irradiated by the reference radiator. Daylight has a high CRI (Ra of approximately 100), with incandescent bulbs also being relatively close (Ra greater than 95), and fluorescent lighting being less accurate (typical Ra of 70-80). Certain types of specialized lighting have very low CRI (e.g., mercury vapor or sodium lamps have Ra as low as about 40 or even lower). Sodium lights are used, e.g., to light highways—driver response time, however, significantly decreases with lower CRI Ra values (for any given brightness, legibility decreases with lower CRI Ra).
Accordingly, for these and other reasons, efforts have been ongoing to develop ways by which solid state light emitters can be used in place of incandescent lights, fluorescent lights and other light-generating devices in a wide variety of applications. In addition, where light emitting diodes (or other solid state light emitters) are already being used, efforts are ongoing to provide light emitting diodes (or other solid state light emitters) which are improved, e.g., with respect to energy efficiency, color rendering index (CRI Ra), contrast, efficacy (lm/W), and/or duration of service.
Although the development of light emitting diodes has in many ways revolutionized the lighting industry, some of the characteristics of light emitting diodes have presented challenges, some of which have not yet been fully met. For example, solid state light emitters are commonly seen in indicator lamps and the like, and in some other types of lighting, but are not yet in widespread use for general illumination.
The most common type of general illumination is white light (or near white light), i.e., light that is close to the blackbody locus, e.g., within about 4 MacAdam ellipses of the blackbody locus on a 1931 CIE Chromaticity Diagram. The 1931 CIE Chromaticity Diagram (an international standard for primary colors established in 1931), and the 1976 CIE Chromaticity Diagram (similar to the 1931 Diagram but modified such that similar distances on the Diagram represent similar perceived differences in color) provide useful reference for defining colors as weighted sums of primary colors.
Because light that is perceived as white is necessarily a blend of light of two or more colors (or wavelengths), no single light emitting diode junction has been developed that can produce white light. “White” solid state light emitting lamps have been produced by providing devices that mix different colors of light, e.g., by using light emitting diodes that emit light of differing respective colors and/or by converting some or all of the light emitted from the light emitting diodes using luminescent material. For example, as is well known, some lamps use red, green and blue light emitting diodes, and other lamps use (1) one or more light emitting diodes that generate blue light and (2) luminescent material (e.g., one or more phosphor materials) that emits yellow light in response to excitation by light emitted by the light emitting diode, whereby the blue light and the yellow light, when mixed, produce light that is perceived as white light. While there is a need for more efficient white lighting, there is in general a need for more efficient lighting in all hues.
In the case of conventional solid state light emitting devices that include one or more luminescent materials, a significant proportion (e.g., in many cases, as much as 20% to 25%) of the excitation light (e.g., light from the light emitting diode(s) that is converted in the luminescent material(s)) is reflected from the phosphor back into the light emitting diode(s) (a phenomenon sometimes referred to as “back-scattering”). Back-scattered light that is scattered back into a light emitting diode has a very low probability of coming out of the chip, and hence, such-back-scattering results in a system loss of energy.
In addition, light that is converted by a luminescent material is often emitted omni-directionally, so that in general, 50% of the light is directed back toward its source (e.g., the light emitting diode).
Furthermore, if the luminescent material(s) is/are contained in a luminescent material-containing element (e.g., a substantially transparent element in which the luminescent material is dispersed), depending on the thickness of the luminescent material-containing element and/or the loading of luminescent material in the luminescent material-containing element, significant “self-absorption” may occur. Self-absorption occurs when light that is absorbed, converted and re-emitted by luminescent material is re-absorbed by luminescent material or otherwise prevented from exiting the luminescent material-containing element, thus reducing performance (intensity) and efficiency.
Various lighting devices have been provided in which light emitting diodes are spaced from luminescent material.
For example, U.S. Pat. No. 5,959,316 (Lowery '316) discloses a semiconductor device which has a light-emitting diode covered by a transparent spacer which separates the LED from a uniformly thick fluorescent material containing layer. Referring now to FIG. 3, therein is shown a lead frame 12 with the reflector 16 which holds the LED 18. A transparent spacer 50 is shown encapsulating the LED 18, and a level of fluorescent material 52 is shown disposed above the transparent spacer 50.
U.S. Pat. No. 6,350,041 (Tarsa '041) discloses a solid state lamp emitting a light that comprises a solid state Light Source which transmits light through a Separator to a Disperser that disperses the light in a desired pattern and/or changes its color. The Disperser 16 can shape or distribute the light in a predefined pattern (such as radially uniform) and it may also contain elements, such as phosphors, fluorescent polymers and/or dyes, to change the wavelength of at least some of the incident light.
Zhu et al., “Optimizing the Performance of Remote Phosphor LED,” First International Conference on White LEDs and Solid State Lighting, contains a statement that the interest for remote phosphor LEDs—where the LED chip and the phosphor layer are physically separated—has been growing. Zhu et al. states that the first remote phosphor LED configuration was proposed in 1995, and that since then, several remote phosphor LED concepts have been proposed. Zhu states that it was not until the middle of the present decade that the benefits of remote-phosphor LEDs were quantified, and that these studies showed improved life and higher luminous efficacy for remote phosphor LEDs compared to traditional PC white LEDs.
U.S. Pat. No. 6,936,857 (Doxsee '857) discloses a visible light emitting device comprising an LED or laser diode and a phosphor. In one arrangement, Doxsee '857 discloses that there is a space 16 between the LED and the phosphor, and such space 16 can be either a vacuum or filled with a transparent gas or solid.
U.S. Pat. No. 6,841,804 (Chen '804) discloses a white light emitting diode device. According to Chen '804, unlike conventional techniques, the reflective approach does not apply the yellow phosphor directly on the blue LEDs. Instead, a layer of the yellow phosphor is applied on the reflector, or alternatively, on a transparent film, which is then attached to the reflector. When the blue light emitted from the blue LEDs is mixed with the yellow light generated by the yellow phosphor stimulated by the blue light, a white light is generated. Then, the reflector reflects the white light to light the area or object. Chen '804 states that as the yellow phosphor is not directly applied on the LEDs, it is not damaged by the heat generated by the LEDs.